The present invention relates to a presensitized plate useful for preparing a lithographic printing plate, whose light-sensitive layer includes a fluoro-aliphatic group-containing copolymer (hereunder also referred to as “fluorine atom-containing polymer” or “fluorine polymer”) and more specifically to a presensitized plate useful for preparing a lithographic printing plate, which may have uniform coating surface without causing any defect such as foaming during preparation of the plate and which can provide a lithographic printing plate excellent in the developing ability and ink-receiving properties.
A presensitized plate useful for preparing a lithographic printing plate has a construction in which a light-sensitive composition (an image-forming composition) is coated on a substrate. A typical production process thereof comprises the steps of applying a light-sensitive composition dissolved or dispersed in an organic solvent onto a substrate, which has been subjected to an appropriate surface treatment and/or which has been provided with an undercoating layer and/or a back coating layer, optionally applying a top coat (or upper) layer such as a protective layer and then drying. In addition, a typical plate-making process comprises the steps of inducing an imagewise change in physical properties of a light-sensitive composition applied onto a substrate by the contact- or projection-type imagewise surface exposure through a mask carrying an image or direct exposure according to, for instance, the scanning or modulation of electromagnetic waves based on image information outputted from a computer, removing (or developing) the light-sensitive layer on the un-exposed areas, followed by, for instance, optional hydrophilization, lipophilization and formation of a protective film to thus form a lithographic printing plate comprising non-image areas or the exposed hydrophilic substrate surface and image areas or the surface of the hydrophobic light-sensitive layer. In a typical printing step, the hydrophilic non-image areas on the lithographic printing plate thus prepared receives dampening water, while the hydrophilic image areas formed thereon receives ink to thus form ink images on the surface of the printing plate. The resulting ink images are directly or indirectly transferred to a desired printing medium to thus form printed matters.
Regarding the light-sensitive layer (image-forming layer) used in such a presensitized plate, there have been known a wide variety of techniques relating to, for instance, light-sensitive layers, which make use of changes in physical properties upon exposure to light, such as negative type one in which the initially soluble type layer is converted into insoluble one upon exposure to light and positive type one in which the initially insoluble type layer is converted into soluble one as well as light-sensitive layers, which make use of, for instance, optical reactions, heat mode processes and heat-sensitive recording as the principle for inducing such physical property changes. To prepare a highly useful presensitized plate for lithographic printing plates, irrespective of the kind of the light-sensitive layer selected, a common technical problem arises. More specifically, (1) the light-sensitive layer should have a high uniformity; and (2) the image area should have a high hydrophobicity and the non-image area should easily be removed through development.
The uniformity of the image area is mainly ascribed to the foregoing preparation process, from the technical standpoint and the presensitized plate insufficient in the uniformity is not preferred since it never satisfies such a basic requirement that the printing plate prepared from the presensitized plate should stably provide a large number of printed matters carrying uniform images of high quality. Moreover, the image area preferably has a high hydrophobicity, since this results in the improvement of the resistance thereof to a developer, ensures excellent resolution in the plate-making step and also permits the achievement of sufficient printing durability and sufficient ink-receptivity. However, extremely high hydrophobicity of the image area may result in the reduction of the solubility thereof in an alkaline aqueous solution as a commonly used developer and this may in turn lead to undesirable results such as the insufficient development of non-image areas and the generation of sludge components in the developer. More specifically, the light-sensitive layer should simultaneously satisfy two requirements contradictory to one another, i.e., the hydrophobicity of the image area and the easy removability of the non-image area. The development of a technique, which can satisfy these two requirements at the same time, is quite difficult and has been an important problem to be solved.
It has been known that the use of a composition containing a fluoro-aliphatic group-containing high molecular weight compound as a light-sensitive composition is quite effective for solving such a technical problem. For instance, Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. Sho 54-135004 discloses that the use of such a composition is effective as a technique for improving the uniformity of an image-forming layer. In addition, J.P. KOKAI Nos. Sho 62-170950, Hei 8-15858 and 2000-19724 disclose the usefulness of the copolymer comprising fluoro-aliphatic group-containing monomer units and specific functional group-containing monomer units. These techniques permit the elimination of the insufficiency of the prior arts, which disclose the use of a fluoro-aliphatic group-containing polymer compound, by the selection of additional substituents. In other words, these techniques are those for reducing the adverse effect of such a fluoro-aliphatic group-containing polymer on the plate-making and printing processes or, contrary to this, those for making the most use of the effect of the polymer. More specifically, J.P. KOKAI Sho 62-170950 discloses further improvement of the film-uniformizing function due to the improvement of the surface activity of the light-sensitive layer; J.P. KOKAI Hei 8-15858 discloses the elimination of the slow developing ability due to the hydrophobicity, while making use of such a polymer; and J.P. KOKAI 2000-19724 discloses the contrast image-forming effect achieved by the simultaneous satisfaction of the two contradictory requirements or the requirements for the hydrophobicity of the image area and the easy removability of the non-image area, while making use of the hydrophobicity-orientating ability.
Among the effects obtained by the use of the fluoro-aliphatic group-containing polymer, the film-uniformizing function depends on the surface-activating capability, that is, the capability of image-forming composition to decrease the surface tension of a dispersion solution of the composition in organic solvent during the preparation of the lithographic printing plate. The other effects obtained by the use of the fluoro-aliphatic group-containing polymer are due to high hydrophobicity of the compound, or other characteristics of the compound such as orientation, deviation and localization to the surface of the image-forming layer. That is, the fluoro-aliphatic group-containing polymer can be relatively distributed to around the surface of the image-forming composition and thus the polymer gives the surface of the image-forming composition especially high hydrophobicity with maintaining the removability of the image-forming layer during the development. Furthermore, it is also possible to improve the fluoro-aliphatic group-containing polymer without altering copolymerizable components. For instance, J.P. KOKAI 2000-187318 discloses that the use of a polymer compound derived from monomers having at least two fluoro-aliphatic groups may provide an image-forming material which shows excellent discrimination between the solubilities of the image and non-image areas.
As discussed above, the use of a light-sensitive layer containing a fluoro-aliphatic compound is effective as a method for achieving the foregoing technical purposes (1) and (2) common to the light-sensitive layer for a presensitized plate used for preparing a lithographic printing plate. On the other hand, however, the effect has still been insufficient and the development of a further improved such technique has still been desired.
For instance, when using a positive type light-sensitive layer, preferably used for preparing good printed matters are light-sensitive layers having a high ability of discrimination between the image and non-image areas or a high gradation (high contrast) obtained after exposing them to light and developing from the viewpoint of the image reproduction and the resistance to defects and there have been desired for the development of a light-sensitive layer having a high sensitivity, an ability of preventing the formation of an indistinct image during printing, safety for the light emitted from an incandescent electric lamp and a high acceptability for development, but there has not yet been developed any satisfactory technique.
In this respect, the term “soft tone image” herein used means that when exposing a light-sensitive layer to light through a step wedge and then developing the exposed light-sensitive layer, the difference between the step number at which images begin to remain on the substrate and that at which the film completely remains thereon is large. On the other hand, the term “high contrast image” herein used means that the foregoing difference is small.
Regarding the term “formation of an indistinct image during printing”, gases are generated due to the decomposition of the light-sensitive material, this in turn leads to the swelling of the litho-film, this further makes the complete contact exposure of the light-sensitive layer impossible and such a phenomenon is accordingly caused. In general, when the clear-sensitivity is set at the same level, the higher the contrast of images, the easier the elimination of the formation of an indistinct image. Moreover, the term “safety for the light emitted from an incandescent electric lamp is defined to be the stability in sensitivity of images upon exposure of a printing plate to light emitted from an incandescent electric lamp such as a fluorescent lamp and in general, the higher the contrast of images, the higher the safety for the light emitted from an incandescent electric lamp. In this respect, the step wedge is a slender rectangular film whose density increases by 0.15 per step and used for determining the relation between the quantity of exposed light and the amount of the light-sensitive layer remaining after the exposure thereof to light and the subsequent development thereof. The term “clear-sensitivity” used herein means the sensitivity observed when an image begins to form after the exposure thereof to light and the subsequent development thereof. In addition, the term “acceptability for development” is used for evaluating any change in the sensitivity of images after the exposure to light and the subsequent development, when the concentration of the developer used undergoes a change and, in general, the smaller the change in the sensitivity, the higher the acceptability for development.
A photo-polymerizable printing plate, which comprises a photopolymerization initiator and a polymerizable double bond-containing monomer and is typical of the negative type lithographic printing plate, in particular, a printing plate highly sensitive to laser beams whose wavelength falls within the visible light range and which can directly be imagewise exposed to a laser beam, the gradation thereof has been soft and therefore, the presensitized plate easily causes fogging due to scattered light and/or reflected light, when imagewise exposing such presensitized plate using an inner drum-type laser plate setter, in which the imagewise exposure is performed by fixing such a printing plate and rotating a mirror at a high speed. In general, the printing durability of a printing plate is improved by exposing to light rays having a high energy, but the aforementioned fogging caused due to scattered light and/or reflected light becomes conspicuous in case of the photopolymerizable printing plate, the quantity of exposed light cannot be increased and as a result, the printing durability of the resulting printing plate cannot be improved. For this reason, it is necessary for the improvement of the printing durability to increase the quantity of light for imagewise exposure and at the same time, to prevent the occurrence of any fogging due to scattered light and/or reflected light. In case of imagewise exposure using a laser beam, the light-sensitive layer is exposed to laser beam for a time on the order of about 1 μs per dot, while the fogging due to scattered light and/or reflected light is a phenomenon in which a light-sensitive material is exposed to extremely weak light rays over a long period of time on the order of several minutes and the light-sensitive layer is thus cured. Therefore, to make the gradation high contrast may be a means for preventing the fogging. If making the gradation of a light-sensitive material high contrast, it is difficult to optically cure the material with weak light rays and therefore, the light-sensitive material exposed to weak light can be removed through development and the use of such a high contrast light-sensitive material would permit the inhibition of any fogging.
In addition, in case of heat-sensitive lithographic printing plate, which is imagewise exposed to, for instance, an IR laser beam, the discrimination between the image and non-image areas is low or such a heat-sensitive printing plate has a low gradation (soft tone). Therefore, the heat-sensitive printing plate suffers from various problems in that a developer easily penetrates into the portion on the plate, which comes in contact with the hand, that some of images are skipped during development and that the resulting printing plate shows a low resistance to external defects.